1. Field of the Invention
The present invention is directed to a method and apparatus for the control of NOx emissions. More particularly, the present invention is directed to an apparatus and improved method of reburning for reducing NOx emissions from combustion systems such as power plants, boilers, process furnaces including glass furnaces, incinerators, and the like. Through the use of high velocity injection of reburn fuel, the present invention provides an apparatus and method for NOx reduction without the need for recirculated flue gas.
2. Technology Review
Combustion of fossil fuels, especially coals and heavy oils, produces a significant amount of NOx which ultimately participates in the formation of smog and acid rain. Combustion modification techniques, including staged combustion and reburning, have been effective in achieving up to 60% NOx reductions.
Reburning is a controlled process which uses fuel to reduce oxides of nitrogen that are collectively referred to as NOx. These oxides include NO (nitric oxide), NO2 (nitrogen dioxide), N2O4 (dinitrogen tetroxide), and N2O (nitrous oxide). In the reburning process, a fraction of the fuel, typically between 10% to 20% of the total heat input, is injected above (i.e., downstream of) the main heat release zone to produce an oxygen deficient reburning zone. The combustion of reburning fuel forms hydrocarbon radicals which react with nitric oxide to form molecular nitrogen, thus reducing NOx. This process occurs best in the absence of oxygen. Subsequently, burnout air is injected downstream to combust the remaining fuel fragments and convert the exiting HCN and NH3 species to either NO or NO2.
Previous studies have shown that 60% reduction in NOx emissions can be achieved with natural gas reburning and that most of the reduction occurs in the reburning zone. NOx reduction in the burnout zone, via the HCN and NH3 species from the reburning zone, can occur depending upon the temperature in that region. As is often the case in most utility boilers, however, NOx reduction in the burnout is minimal because of the high burnout temperature (2200xc2x0-2400xc2x0 F.) and the presence of an excessive amount of carbon monoxide (xe2x80x9cCOxe2x80x9d, above 2% at 0.9 stoichiometry).
The overall reburning process can be divided conceptually into three zones as follows:
Primary Zone: This main heat release zone normally accounts for approximately 80 percent of the total heat input to the system and is operated under fuel lean conditions. The level of NOx exiting this zone is defined to be the input to the reburning process. If sufficient residence time is not provided, unburned fuel fragments may leave this zone and enter the reburning zone.
Reburning Zone: The reburning fuel is injected downstream of the primary zone to create a fuel rich, NOx reduction zone. Reactive nitrogen enters this zone from two sources: the primary NOx created in the main heat release zone and the fuel nitrogen, if any, in the reburn fuel. These reactive nitrogen species react with the hydrocarbon fragments formed during the partial oxidation of the reburning fuel, primarily CH species, to produce intermediate species such as HCN and NH3. Additionally, some nitrogen is converted to N2 and some is retained as NO. If the reburning fuel is a solid such as coal, nitrogen may also leave this zone as char nitrogen.
Burnout Zone: In this final zone, air is added to produce overall lean conditions and to oxidize all of the remaining fuel fragments. The total fixed nitrogen species (TFN=NH3+HCN+NO+char nitrogen) will be either oxidized to NOx or reduced to molecular nitrogen.
The application of reburning technology to industrial and utility combustion systems requires a technique for effective entrainment and penetration of the reburning fuel, such as natural gas, with the combustion products from the main combustion zone, in order to achieve the highest overall NOx reduction. This is difficult to achieve because it requires the mixing of a relatively small amount of reburn fuel (typically between two to three percent of the total fuel mass input) with flue gas having a relatively much larger cross-section (in comparison to the area of the introduced stream of reburn fuel). It will be understood, therefore, that it is particularly difficult to achieve the desired entrainment and penetration of low mass reburn fuels, such as natural gas. This problem is exacerbated when the source of reburn fuel is provided at a low flow rate.
In conventional systems, satisfactory penetration and entrainment of the reburning fuel have been achieved by increasing the momentum of the reburn fuel jet, which is typically provided at a low source flow rate, by introducing recirculated flue gas through the reburn fuel nozzles. The flue gas is typically fed to the reburn fuel injectors under pressure, by way of a blower, compressor, or the like. Alternatively, flue gas may be educted at the reburn fuel port. In this manner, the reburning fuel is injected with sufficient force to achieve adequate penetration into, and entrainment with, the combustion effluents.
The use of flue gas recirculation to enhance the mixing of reburn fuel is expensive, however, both in terms of the capital expenditure that is necessary to implement such a system, as well as in terms of significantly higher operating costs. In this regard, a flue gas recirculation system requires sufficient feed lines and a fan or blower system to transport the flue gas to the reburning nozzles from a point downstream of the reburning zone. The use of flue gas recirculation is also undesirable because it requires the introduction of additional reburn fuel to consume the additional oxygen that is present in, and carried with, the stream of recirculated flue gas.
It is therefore apparent that there remains a need for an improved means for the injection of reburn fuel without the need for recirculated flue gas or other carrier gas.
An apparatus and method are disclosed for high velocity injection of a reburn fuel into a stream of NOx containing combustion effluents above (i.e., downstream of) a primary combustion zone, without the use of recirculated flue gas or other carrier gas. The apparatus includes a fuel introducing member that has a fuel receiving end for receiving fluid fuel from a fuel source and a fuel injection end for injecting the fluid fuel into the stream of combustion effluents. A fuel passage is in fluid communication with the fuel receiving end and the fuel injection end of the fuel introducing member. A mechanism for increasing the velocity of the fluid fuel, without the need of a carrier gas, is associated with the fuel introducing member.
In accordance with one embodiment of the apparatus, the velocity increasing means comprises a nozzle with an end plate that has one or more holes. The cross-sectional area of the one or more holes of the nozzle end plate is less than the cross-sectional area of the fuel introducing device. Because of this differential in cross-sectional areas, the fluid fuel, which is typically provided at a low velocity source rate, exits the nozzle end plate at a significantly higher velocity (which may be at or, depending upon the nozzle structure, above sonic speeds).
In accordance with another embodiment of the apparatus, the velocity increasing means comprises a blower or compressor that is associated with the stream of reburn fuel and is located upstream of the fuel introducing member. In accordance with still another embodiment of the apparatus, the velocity increasing means comprises a constricted portion of the fuel passage of the fuel introducing member or, alternatively, a constricted portion of the upstream portion of the fuel supply line. In this latter embodiment, the portion of the reburn fuel system that is downstream of the constricted portion of the fuel supply line is generally of a size that maintains the same or a comparable cross-sectional area as the constricted portion of the fuel supply line.
Also disclosed is a method of injecting a fluid fuel into stream of combustion effluents containing NOx, above (i.e., downstream of) a primary combustion zone, at a high velocity. The method comprises the steps of:
providing a source of fluid fuel;
providing one or more fuel injectors, the one or more fuel injectors each comprising a fuel introducing member, the fuel introducing member having a fuel receiving end for receiving fluid fuel from a fuel source, a fuel injection end for injecting said fluid fuel into the stream of combustion effluents, and a fuel passage that is in fluid communication with the fuel receiving end and the fuel injection end;
providing a means for increasing the velocity of the fluid fuel without the use of a carrier gas;
positioning the fuel injector at a location above (i.e., downstream of) the primary combustion zone of the effluent source; and
providing a means of fluid communication between the source of fluid fuel and the one or more fuel injectors.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.